Pulmonary vasoconstriction and vascular medial hypertrophy greatly contribute to the elevated pulmonary vascular resistance in patients with pulmonary hypertension. A rise in cytosolic free Ca2+ ([Ca2+](cyt)) in pulmonary artery smooth muscle cells (PASMC) triggers vasoconstriction and stimulates cell growth. Membrane potential (E(m)) regulates [Ca2+](cyt) by governing Ca2+ influx through voltage-dependent Ca2+ channels. Thus intracellular Ca2+ may serve as a shared signal transduction element that leads to pulmonary vasoconstriction and vascular remodeling. In PASMC, activity of voltage-gated K+ (Kv) channels regulates resting E(m). In this study, we investigated whether changes of Kv currents [I(K(V))], E(m), and [Ca2+](cyt) affect cell growth by comparing these parameters in proliferating and growth-arrested PASMC. Serum deprivation induced growth arrest of PASMC, whereas chelation of extracellular Ca2+ abolished PASMC growth. Resting [Ca2+](cyt) was significantly higher, and resting E(m) was more depolarized, in proliferating PASMC than in growth-arrested cells. Consistently, whole cell I(K(V)) was significantly attenuated in PASMC during proliferation. Furthermore, E(m) depolarization significantly increased resting [Ca2+](cyt) and augmented agonist-mediated rises in [Ca2+](cyt) in the absence of extracellular Ca2+. These results demonstrate that reduced I(K(V)), depolarized E(m), and elevated [Ca2+](cyt) may play a critical role in stimulating PASMC proliferation. Pulmonary vascular medial hypertrophy in patients with pulmonary hypertension may be partly caused by a membrane depolarization-mediated increase in [Ca2+](cyt) in PASMC.